It is known that slippery ice occurs at temperatures between +2.degree. and -8.degree. C, according to the relative humidity of the air. The heaviest snow falls also occur in this temperature range. Below -8.degree. C, the humidity of the air is too low for precipitation on the road surface and snow falls are scanty.
Heated road surfaces have already been used, in particular on bridges, for preventing the formation of ice and for melting snow, the heating being produced by electrical resistors embedded in the road surface or by means of embedded pipes through which hot water flows. Arrangements of this type are naturally quite expensive and require energy which has to be supplied externally.
It is also customary to spread hygroscopic salt, for example calcium or sodium chloride, on the road surface to lower the freezing point of water. However, the effect of spreading salt generally has a very short duration, since as vehicles pass by, they push the salt to the edge of the road and the water containing the salt drains away from the road.
The incorporation of a thawing agent in a road surface has already been proposed in British Patent Specification No. 433957, which previewed embedding particles of anhydrous magnesium chloride in a solid or plastic medium which allows only a gradual access of moisture to the thawing agent. This plastic medium, forming a binding agent, was preferably bitumen of the road surface, or alternatively wax, paraffin and the like. However tests carried out on this product in a sandwich road construction, reported in the publication Deutches Strassendienst (1958), showed that to achieve any effect, the road surface should have in excess of 50% by weight of magnesium chloride. During the first winter of testing the effect was poor and during the second winter there was no effect. The cost of building the road was increased by 50% and no way of improving the performance and longevity was found. Consequently, the idea was dismissed as being promising on paper, but not practical or economic.